Devices are known for the detection of radiation signals which may be used, for example, to determine the gas concentration, using infrared absorption. Interfering gases in a gas volume, such as in the passenger compartment of a motor vehicle, are, for example, CO2, CO, H2O or even CH4, the carbon dioxide CO2 originating from especially the air breathed by people located in the gas volume and also from CO2 air conditioning systems. Such interfering gases are detected by generally known gas detectors. Such detectors for the selection of gases, based on the principle of radiation absorption in the measured gas, are usually made up of a radiation source and one or more wavelength-specific radiation detectors. For sensors which measure the absorption in the medium infrared range, such as in the range of wavelengths of 3 μm to 25 μm, such as sensors for the detection of carbon dioxide CO2, usually thermal radiators are used as the source. In this connection, there are infrared sources in the form of incandescent lamps or in the form of microstructured devices that function as thermal radiators. Wavelength-specific radiation detectors are made up of a filter unit which transmits the wavelength to be measured and a broad-band infrared detector. Interference filters are used in this connection, for example. Simple interference filters have the property of also transmitting harmonics of the desired wavelength. This may be rectified, first, by a more complicated and, thus, more cost-intensive construction of the interference filter, or also by putting a simple broadband filter before the actual detector. If such a broadband filter is used, this conditions a costly construction of the detector, since in that case two filters have to be mounted.